Wire wound element



Aug. 27, 1957 BQURNS Er AL 2,804,529

WIRE WOUND ELEMENT Filed May 1, 1953 I N VEN TORS MARLAN E. BOURNS BYMARViN E. HARRISON WIRE WOUND ELEMENT Marlan E. Bourns and Marvin E.Harrison, Riverside, Calif.; said Harrison assignor to said Bourns Thepresent invention relates to potentiometer elements, and moreparticularly to that type of precision potentiometer element made bywinding resistance wire on a molded form or card.

In the making of wire wound resistance elements used in rheostats orpotentiometers, it is customary to wind the wire on flat forms or cardsproduced by sawing strips from a sheet of plastic material and radiusingthe edges of these strips to form rounded corners. This method has manylimitations inasmuch as the form is necessarily rectangular when viewedfrom the side, with no steps in it. Also, it is difficult to producebecause a sawing or grinding operation is involved. Furthermore, sinceit is important that extremely fine surface finish and parallelism bemaintained in order that the finished element will be linear inelectrical output, the tolerances involved often become difficult toachieve.

The primary object of our invention is to provide a new and improvedcard for wire wound resistance elements, which is easy and inexpensiveto manufacture, and which can be produced to extremely tight tolerancesof dimension and parallelism, as well as having a surface finish orsmoothness that is adequate to permit the smooth winding of extremelyfine wire, of the order of .005" to .0005, with no gaps or cross-overscaused by irregularities in the surface finish of the card itself.

It is a further object of our invention to produce a wire woundresistance element having a card which may be molded offiber-glass-filled phenolic resin that will withstand very hightemperatures. This material is not readily available in sheet form, andif obtained, is difficult to machine and to cut into strips. It is alsovery difiicult to machine a radius and a smooth finish on the edges ofsuch a material after it has been cut, because of the glass fibers thatare embedded therein.

It is a further object of our invention to provide a wire woundresistance element embodying a card molded of rubber or other flexiblematerial, whereby the finished element may be bent in an are withoutdamaging the element or card.

Another object of our invention is to provide a stop which is moldedintegrally with the element to prevent the wiper from traveling beyondthe end of the element and lodging at this point, such that its returnto the wiped element surface is impossible.

A further object is to provide a wire wound resistance element havingresistance wire wound into a long cavity of such a depth that if thecontact which rides on the wire-wound surface goes beyond the end of theactual winding, it will travel on the step molded into the card, and thejunction between the end of the wound surface and the surface of thecard itself will be smooth and will not cause the contact to hang up orstick at this juncture.

Another object of the present invention is to provide a wire woundresistance element having a cavity into which a shorting strip may bewrapped, so that the finished height of the shorting strip will be thesame distance ates Patent ice above the remainder of the card as thediameter of the wire with which the element is wound, thereby providinga smooth juncture between the wire and the metal shorting strip. Thispermits the use of rather thick shorting strip material which is verydurable and readily handled, rather than extremely thin material equalin thickness to the diameter of the wire, which would otherwise berequired if the card had no step molded into it.

A further object of this invention is to provide an element which hasmetal shorting strips molded integral with the card itself. These stripsproject above the edge of the card such that the winding is flush withthe shorting strip surface. These metal portions molded integral withthe card may also be used as a termination means, since the element wireand also the electrical lead wire may both be spot welded or otherwiseattached to them.

Still a further object of our invention is to provide a card having acavity of predetermined length formed in one edge thereof, so that thewire with which the element is wound can readily be peeled bac to therequired winding length without the necessity of making accuratemeasurements by means of a scale or special termination device.

The various features of novelty which characterize this invention arepointed out with particularity in the claim annexed to and forming apart of the specification. For a better understanding of the invention,reference may be had to the accompanying drawings, which illustratecertain preferred embodiments of the invention. In the drawings:

Figure 1 is a partially cut-away elevational view of a Wire woundresistance element embodying the principles of our invention;

Figure 2 is a similar view of a modified form of the invention;

Figure 3 is a transverse sectional view taken at 3'3 in Figure 2;

Figure 4 is an elevational view of a third form of the invention; and

Figure 5 is a cross-sectional view taken at 55 in Figure 4.

Figure 1 shows a molded plastic element card 11 having a cavity 12 intowhich fine resistance wire 13 has been wound. The card 11 may be made ofany suitable plastic material, although we have found that excellentresults are obtained with fiber-glass-filled phenolic resin, which isthermo setting and has satisfactory dielectric properties. Other thermosetting plastics, such as urea, melamine, and polyester resins may beused, either with or without fillers such as fiber glass, wood flour,and rubber; or if desired, certain of the thermo plastic resins such aspolyethylene and polyamide may be used. The card 11 is an elongated,flat, relatively thin strip, with radiused top and bottom edges in thecavity 12. It may be produced by any suitable molding process, such ascompression or transfer-compression molding, and comes from the mold inthe desired shape with any necessary inserts embedded therein.

At the ends of the card, beyond the wire windings, the edges of the cardare stepped up at 14 to provide surfaces fiush with the top surface ofthe wire, onto which the contact 15 may ride when it travels from theend of the wire wound surface 13 onto the remainder of the card. Thecontact 15 is a part of the variable resistor or potentiometer withwhich the resistance element is assembled, and is shown herein only forthe purpose of illustrating the manner of operation. In the event thecontact 15 travels beyond the normal excursion, it engages a limit stoppin 16, which is molded into the edge of the card as an insert. Thisprevents the contact from going beyond the end of the element andhanging up. Holes 17 are molded into the card adjacent the ends thereoffor usein inserting termination rivets for fastening the element wireand the'lead wire."

When Winding the element shown in Figure 1, it is customary to windnearly to the ends of the card, then peel back the resistancejwire,"sincethe' beginning "and" ending of the winding is frequently oflo'werquality winding than the central portion". Peeling 'b'a'ck mayeasily be done, since the wire may merely bepeeled back until the cavity12 is reached. This provides an easy and automatic means of stopping atthe'desired place and thereby providing the required winding length.

Figures 2 and 3 show another embodiment of our invention. The card 18has a recess 19 molded into one edge thereof beyond the end of the"wirewindings, and bent over the said recess is a shorting strip 20',which is secured in place by a terminal rivet '21. The bottom of thenotch 19 is set down below the surface on which the wire 22 is wound, sothat thetop surface 'of the shorting strip 20 is level with'the topsurface of the wire 22. For example, if the wire 22 is .005" diameter,and the shorting strip 20 is .015" thick, the recess 19"would be .010"deep. Thus, the contact 23 may pass over the juncture between the wireand the shorting strip With' no irregularity being encountered.Anupwardly projecting limit stop 24 is molded integral with the cardbeyond the recess 19, to prevent the contact 23 from traveling beyondthe end of the shorting strip 19. A

terminal rivet 25 is molded into the card 18 and is used surface of theplastic strip 26 on which the wire 33 is' wound, so that the juncture ofthe wire 33 with the shorting strips 27 will be smooth. Hence,thecontact 29 may travel beyond the end of the windingonto" the shortingstrip with no irregularity or discontinuity. The element wire 33 is spotwelded or otherwise attached to the shorting bar 27 at the point 30;likewise thelead wire 31 is similarly attached at the point 32. Theshorting bars 27 are securely anchored into the plastic material bymeans of holes 28, which permit the plastic to flow through from bothsides.

In still another aspect of the invention, we take advantage of themolding process of forming element cards to anchor the wire windingssecurely to the card. This is done by partially curing thethermo-setting resin under heat and pressure, to the point where thecard is still slightly soft, and then removing the card from the mold.The partially cured card is wound with the wire windings, and thenheated to complete the cure of the resin. As the wire is wound onto thesoft card under tension, it tends to sink into the soft plastic, andthus becomes partially embedded therein. When the card is fully cured,the Wires are firmly adhered to it, and will not slip along the cardunder the thrust of the wiper contact.

While We have shown three illustrative embodiments of our invention, itwill be understood that We do not Wish to be limited thereto, as variouschanges and modifications will occur to those skilled in the art, whichcome within the scope of the appended claim.

We claim:

A wire-wound electrical resistance element comprising a card offiber-glass-filled phenolic resin molded into an elongated, relativelyflat strip, a plurality of turns of resistance wire wound on said card,said card having a recess molded into one edge thereof beyond the end ofsaid Wire, a shorting strip bent over said one edge of said card andseated within said recess, said shortingstrip being connected to theadjacent end of said wire and being of a thickness such that the outeredge of said shorting strip is flush with the outer edge of said wireturns, and a limit stop molded integrally with said card beyond the endof said shorting strip and projecting for a substantial'distance abovesaid outer edge thereof.

References Cited in the file of this patent UNITED STATES PATENTS822,332 Wood June 5, 1906 1,433,740 Nagel Oct. 31, 1922 2,307,626 KellyJan. 5, 1943 2,311,613 Slayter Feb. 16, 1943 2,332,376 Haberberger Oct.19, 1943 2,346,598 Mucher Apr. 11, 1944 2,371,313 'Rast et al Mar. 13,1945 2,408,093 Patterson Sept. 24, 1946 2,468,144 Van Alen Apr. 26, 19492,471,592 Thomson May 31, 1949 2,534,994 Scott et al. Dec. 19, 19502,566,911 Scott Sept. 4, 1951

